Energy drink (ED) use, particularly when mixed with alcohol (AED), is a rapidly emerging phenomenon that has been linked with both problem drinking and unsafe sexual activity. Although these drinks have been widely available in the United States for more than a decade, the parameters and implications of their use remain significantly understudied. While a few preliminary studies have examined ED and AED use in regional convenience samples of college students, little is known about patterns of use among adolescents and young adults nationwide. The present R01 application has three key objectives. First, we will collect the first detailed, nationally representative data to map the prevalence and demographic distribution of ED and AED use in U.S. minor adolescents (aged 13-17) and emerging adults (aged 18-25). Variations across gender, race/ethnicity, age, college enrollment status, sports involvement, and other key characteristics will be assessed. Second, we will examine links among AED use, AED expectancies, and sexual risk-taking in emerging adults. Both event- level and prospective associations will be assessed. We will control for demographic and personality characteristics as well as other alcohol and/or caffeine use, in order to examine the unique effects of AED use on sexual risk-taking (i.e., multiple and/or casual partners, sexual intercourse while intoxicated, and lack of protective behaviors like condom use). We will draw on expectancy theory to test whether beliefs about the interaction of alcohol and caffeine moderate the relationships between AED use and sexual risk behaviors. Third, we will test for gender differences in these relationships. The proposed study will recruit demographically diverse national probability sample of 3,000 U.S. youth, including 2,000 emerging adults aged 18-25 and 1,000 minor adolescents aged 13-17. Six waves of web survey data on ED and AED use, alcohol use, and sexual risk-taking will be collected at three-month intervals from the emerging adult participants. Minor adolescents will complete a less-sensitive survey on ED and AED use in Wave 1 only. By providing the first national data addressing these questions in detail, the research will provide a theoretically coherent and empirically sound basis for understanding the complex relationships between the nascent ED/AED phenomenon and adolescent and young adult health. Research findings are intended to locate ED and AED use within a broader pattern of health-risk behavior and to inform the future development of more effective screening, intervention, and regulatory strategies for reducing AED-related risky sexual activity.